Antibiotic resistance in bacterial populations has followed close upon the heels of the widespread use of antibiotics.
As antibiotic-resistant bacteria thrive and sensitive bacteria die, the proportion of resistant cells in a population increases dramatically. These resistant cells continue to undergo binary fission, resulting in an infection that is difficult to treat.
The ability of bacterial populations to evolve resistance requires genetic variation.
Mutation is one source of genetic variation. The large population sizes and rapid reproduction of bacterial populations means that even with a low mutation rate, a significant number of mutations are seen in bacterial populations. Some of these mutations contribute to antibiotic resistance.
Processes such as conjugation allow for genes to be exchanged among bacteria. Because conjugation can occur between bacterial cells of the same species or of different species, antibiotic resistance sometimes spreads surprisingly easily from one species of bacteria to another.
A single R plasmid may have genes that provide resistance to ten or more different antibiotics. This is especially problematic when bacteria that are serious health threats become resistant to multiple antibiotics. For example, methicillin-resistant Staphylococcus aureus (MRSA) infections are caused by bacteria that have evolved resistance to most of the antibiotics that were once used to treat staph infections. MRSA is becoming increasingly difficult to treat.